1. Field of the Invention
The present invention relates to a power tool, such as a disc grinder, an electric screwdriver, or a drill for boring, which is equipped with an electric motor therein as a power source.
2. Description of the Related Art
Such a power tool is generally equipped with either a gear train for changing the number of output revolutions of a motor or a gear train for changing the output direction. A CVT (Continuously Variable Transmission) that continuously varies the gear train and reduction ratio is commonly used as a transmission mechanism for power tools. Technology concerning CVT traction drives are disclosed, for example, in JP No. 6-190740 A, JP No. 2002-59370 A, and JP No. 3-73411 B2.
In a continuously variable transmission traction drive, a plurality of conical planetary rollers are supported by a holder. A centrally located sun roller is pressed onto the planetary rollers. A shift ring located around the holder is pressed onto the planetary rollers. Through rolling contact, planetary rollers transmit rotational power to an output shaft. The number of output revolutions is continuously altered due to the changing of the position of the shift ring relative to the planetary rollers. The pressing position of the shift ring pressed to the conical surfaces of the planetary rollers is varied between a small diameter and a large diameter.
A screw-tightening tool equipped with a continuously variable transmission therein is disclosed in JP 6-190740 A. In the screw-tightening tool, it is possible to continuously vary the speed and torque output. This is accomplished by moving a shift ring. In creating low speed/high torque output, thread-fastening can be easily performed.
Embodiments of a power tool that varies the number of rotations of a driving motor by using a reduction mechanism having a fixed reduction ratio are disclosed. They typically include a sequential transmission mechanism or a continuously variable transmission mechanism, which uses a gear train, and transmits rotation to a front tool.
When an electric disc grinder is used, it may be preferable that the grindstone be rotated at a low speed in order to prevent the scattering of grinding powder and grind water. In other situations it is difficult to create a large reduction ratio in transmission mechanisms.
Therefore, there exists the need to create a power tool, such as a disc grinder, having a large transmission width relative to the gear train. Alternatively, a transmission traction drive mechanism is desired.